1. Field of the Invention
This invention relates to a method and apparatus for manufacturing semi-conductive high-purity silicon rods by subjecting monosilane to pyrolysis on red-heated, elongated silicon carrier members thereby depositing high-purity silicon thereon.
2. Description of the Prior Art
Semi-conductive, high-purity silicon rods have heretofore been manufactured by pyrolyzing or reducing by hydrogen a gaseous silicon compound such as, for example, monosilane, silicon tetrachloride, trichlorosilane or the like, which has been refined on rod-shaped and red-heated silicon carrier members or on a high-melting point metal having good electric conductivity such as, for example, tantalum wire, thereby depositing high-purity silicon on the carrier members. Such representative method and apparatuses for producing high-purity silicon rods are already disclosed in U.S. Pat. Nos. 3,099,534, 3,011,877, 3,053,638 and 3,147,141. However, in the case of subjecting the above-mentioned monosilane (SiH.sub.4) to pyrolysis thereby producing high-purity silicon rods, there occurs a reaction of decomposition inherent in monosilane which can not be observed in the decomposition reaction of halides (SiCl.sub.4, SiHCl.sub.3 etc.), and therefore problems as mentioned hereinbelow) arose even when the above-mentioned method and apparatus are employed.
The reaction of decomposition of monosilane at an elevated temperature differs from that of halides at a high temperature, and the most outstanding difference between them exists in homogeneous reaction speed in gaseous or vapor phase. Monosilane is thermally decomposed by either homogeneous reaction in gaseous phase or heterogeneous reaction, and the reaction required for manufacturing silicon rods is mainly the heterogeneous reaction which takes place on the surface of the red-heated silicon rods. The higher the density and temperature of monosilane in gaseous phase, the higher the speed of homogeneous reaction in gaseous phase and heterogeneous reaction. The dependance of the reaction speed on temperature conditions in the homogeneous reaction in gaseous phase is far greater than that in the heterogeneous reaction. According to a chemical engineering observation, such phenomenon is promoted with the increase of the diameter of high-purity silicon rods which are being manufactured so that the growth speed of silicon on the opposite faces of the red-heated silicon rods will increase as the diameter of the silicon rods. As a result, the diameter of the silicon rods in the cross-section thereof becomes more uneven.
Whilst, the silicon rods used in the float zone melting process are usually molded to have a uniform diameter or approximately perfect roundness by scraping off uneven parts of the silicon rods with a view of preventing occurrence of possible accidents resulting from the irregular shape thereof during the float zone melting process. As mentioned above, the unevenness in the diameters of silicon rods in the cross-section thereof tends to reduce the yield thereof when used as the raw material for single-crystal silicon. Furthermore, with the increase of the diameter of each red-heated silicon rod, besides the deposition of silicon on the surface of growing silicon, the amount of deposition of powdery silicon on the interior wall of the pyrolysis container will increase. The powdery silicon is produced by the aforementioned homogeneous reaction in gaseous phase, and it is envisaged that as the diameter of each of the red-heated silicon rods increase, the temperature of the gaseous phase increases correspondingly so as to promote the homogeneous reaction in gaseous phase. As a result, with the increase of the homogeneous reaction in gaseous phase, the heterogeneous reaction necessary for the manufacture of high-purity silicon rods will increase, and therefore not only the yield thereof is reduced, but also the powdery silicon sometimes commingles with the growing high-purity silicon rods, thus reducing the quality of the latter as the raw material or element for float zone melting.